A photoacoustic tomography (hereinafter, also referred to as “PAT”) apparatus is known as one of apparatuses for visualizing in-vivo information. In the measurement using a PAT apparatus, an image can be obtained by measuring the intensity and the time of generation of a photoacoustic signal emitted from a substance (optical absorber) that absorbs light in an object to be measured when the object is irradiated with light, and computing a distribution of the substance in the object.
Any substance that absorbs light and emits an acoustic wave in a living body may be used as an optical absorber. For example, a blood vessel or a malignancy in the human body may be used as an optical absorber. In addition, for example, molecules of indocyanine green (hereinafter, also abbreviated as “ICG”), may be administered into the body and used as contrast agents. ICG well absorbs light in the near-infrared wavelength region, the light having little influence on the human body when the human body is irradiated with the light and having a high permeability to a living body. Thus, ICG may be used as a contrast agent in PAT apparatuses. In this specification, ICG indicates a compound represented by formula (1) described below.

The counter ion may not be Na+. Any counter ion, e.g., H+ or K+, may be used.
However, it is known that ICG has a short half-life of about several minutes in blood. NPL 1 reports a case of photoacoustic imaging of cerebral blood vessels of a rat with free ICG. According to this report, the photoacoustic signal intensity is reduced to a level equal to that of blood several tens of minutes after free ICG is administered in blood. This suggests that the administered substance is rapidly cleared from blood after administration.
As described above, free ICG is cleared from blood several tens of minutes after administration in blood, thus possibly resulting in a low tumor accumulation when time has passed since administration.